1. Field of the Invention
The present invention relates to the field of automotive-type rectifier assemblies used to convert multi-phase alternating current to direct current using semiconductors located in the rear housing of the alternator. More particularly, the invention deals with the high power requirements and high under-hood temperatures associated with modern day automotive electronics. Further, the invention relates to the complex manufacturing problems created by the high power and high temperature requirements, and also relates to the extreme sensitivity of semiconductors to heat, thermal stress, compression and mechanical forces created during the manufacturing process and generated during long-term operation of the alternator.
2. Description of the Related Art
It should be understood that the power requirements for charging a storage battery alone are approximately 50 amperes or greater. In addition, power is needed to run the air conditioning, the head lamps, an onboard computer, a stereo system, and fans in the engine compartment and in the passenger compartment. Thus, the overall power consumption can exceed 70-90 amperes. The heat generated by the rectifier assembly in producing this much power must be rapidly dissipated in order to avoid breaking down the semiconductor material in the rectifier assembly. This is particularly true during the summer when the ambient temperatures are quite high so that the ambient air does not provide a significant cooling effect. Insufficient cooling of the rectifier assembly typically causes short-term life of the rectifier assembly and ultimately results in high cost repairs or replacement of the alternator.
The present invention is also concerned with automated manufacturing and installation of rectifier assemblies using semiconductor diodes and stamped-out heat sinks, terminals, gaskets and molded parts. The semiconductor diodes are extremely sensitive to thermal and mechanical stresses and forces typically associated with high volume manufacturing techniques, whereas the invention described herein avoids the stresses by gently placing and locking the sensitive semiconductors in place for a one-time riveting and soldering operation, while never exceeding the technical and handling specifications set forth by the manufacturers of the semiconductors.
The prior art teaches that poly-phase alternating current can be converted to direct current suitable for automotive use by using six semiconductor chips or button-type diodes, and by connecting the cathodes of three of the diodes to a positive D.C. heat sink, and by connecting the anodes of the remaining three diodes to a negative D.C. heat sink. The anode of one of the diodes on the positive heat sink is connected by a copper terminal to a cathode of a diode on the negative heat sink, thereby forming a set of diodes in series with the positive and negative D.C. heat sinks. A wire lead extending from phase one of the alternator poly-phase stator winding is connected to the series copper terminal of the first set of diodes. A phase two lead and a phase three lead also extend from the poly-phase stator windings and are similarly connected to the next two sets of diodes to complete the poly-phase series circuit through the positive/negative D.C. power output terminals and the storage battery charging system, as seen, for example, in FIG. 10.
Most rectifier assemblies use semiconductor diodes in their chip form, as illustrated by U.S. Pat. No. 4,606,000, assigned to General Motors Corporation. The chips are only 0.180".times.0.012" in size in the ceramic form with glass passivated edges. The chips are extremely difficult to handle. Nickel-plated copper tabs, which are slightly larger than the chips, are soldered to the anodes and cathodes to strengthen them for the manufacturing process and for the thermal stresses which are incurred during normal operation.
The anodes of three of these chips are affixed to the semicircular copper negative D.C. heat sink. The cathodes of three other chips are affixed to an aluminum casted positive D.C. heat sink. The positive D.C. heat sink is much thicker than the negative heat sink and has a series of cooling slots which are extended out into the alternator's cooling airflow area. The positive heat sink is mounted above the negative heat sink, and separated by a very thin silicone/fiberglass woven insulation gasket. Three flat thin copper complex-formed terminal strips are formed to connect the stator leads to the diodes. Each terminal strip has three legs extending multi-directionally out of a plastic molded dove-tailed insulating support member. The insulating support member is press-fit into the three matching slots which are machined on the peripheral surface of the aluminum heat sink. The support members are then staked into position. Six of the thin copper legs are "U"-formed to align with and be affixed to the three sets of diodes located on the positive and negative heat sinks, respectively. The remaining three legs connect to the stator windings during alternator installation to form a series circuit through the poly-phase stator windings and the diodes. Prior to the alternator installation, the rectifier assembly is processed through multiple solder applications and solder furnace temperature stages to solder the semiconductors to the heat sinks. During this process, the semiconductors are held in position by a slight axial force from the copper "U" stator terminal above them. After the multiple stages of soldering are completed, and the noise suppressant capacitor is staked into position, a plastic cover is pushed into position and silicone rubber is injected around the semiconductor chips and capacitor to protect them from environmental hazards, such as salt spray, dust and metal particles. The rectifier assembly is then tested and installed into the alternator.
The prior art devices cannot transfer the heat generated by the diodes to the cooling air flow air rapidly enough to prevent thermal damage during high ambient temperatures because the conventionally used aluminum heat sink has approximately half the thermal conductivity of copper. An alternate thermal cooling path through the copper heat sink under the aluminum is further decreased by the silicone/fiberglass woven "insulator" gasket.
The top surfaces of the semiconductor chips soldered to the stator "U" terminals also cannot dissipate the heat of the semiconductor chips to the cooling air because the semiconductor chips are encapsulated in an insulating silicone after the plastic cover is installed. Thus, during high ambient summer temperatures and power loads, the semiconductors are thermally overloaded, causing premature alternator failures.
The prior art also presents high volume manufacturing problems associated with placing, locating and holding the extremely sensitive semiconductors in position for the soldering operation. The semiconductors are extremely light, and they tend to float up and around because the thin copper "U"-shaped stator terminals required to hold the semiconductors in position have minimal or no axial force. There is also a high cost associated with the ultrasonic welding process required to weld the copper diodes tabs to the aluminum heat sink because copper cannot be soldered to aluminum.
The thin silicone/fiberglass woven insulator gasket under the aluminum also presents major problems during installation of the rectifier assembly into the alternator. If the installation bolts are slightly over-torqued, the sharp edges of the aluminum heat sink will cut through the thin gasket to create a direct short circuit to the negative heat sink. The over-torquing may result in the positive plate shorting out to the negative plate. The gasket under the aluminum must be both thermally conductive and "thin" to allow the heat generated by the positive diodes on the aluminum to rapidly conduct into the copper heat sink and alternator housing, and ultimately into the ambient air.
If slightly under-torqued, the loss of thermal conductivity will cause the rectifier to overheat or become loose and to fail in the field. Even if the rectifier is installed at the recommended torque, the alternator will prematurely fail because of the high coefficient of expansion of the aluminum. The aluminum expands each time the engine is started and the temperature rises above 300.degree. F., and the aluminum contracts when the engine is turned off and the temperature returns to the ambient temperature, which could be below 0.degree. F. The repeated expansion and contraction of the aluminum positive heat sink also causes the rectifier assembly and the positive electrical output connection on the alternator housing to become loose. Once loose, the rectifier assembly begins to overheat, and the overheating results in the premature failure of the alternator.
Electrical problems inherent with aluminum have been well documented in the building industry. Most electrical codes have banned aluminum wiring because the terminal connections on the aluminum wire become loose, overheat and cause fires. Similar problems can occur with the use of aluminum connections in automotive rectifier assemblies.
Other rectifier manufacturers have attempted to solve the prior art manufacturing problems using the same semiconductor chips on the same aluminum and copper heat sinks with the same thin silicone/fiberglass insulator gasket separating the heat sinks. The other manufacturers merely replaced the dove-tailed, complex bent copper stator terminals with short 0.032" round copper molded into the plastic cover and extending downward with a thin, flat copper "S" termination instead of a "U" termination to apply an axial force against the semiconductor chips to hold the chips in place for the multiple soldering and encapsulation process. Consequently, the rectifier assemblies overheat, become loose and fail in the same manner as prior rectifier assemblies.
Still other manufacturers have tried replacing the glass passivated semiconductor chips (which were soldered on top of the aluminum and copper heat sinks) with "button"-type diodes. The three positive diodes were placed in three wells in the same type of aluminum heat sink with their respective anodes protruding above the top of the wells. The other three button diodes were placed on the same type of copper heat sink with their respective cathodes in alignment with the anodes of the diodes in the aluminum wells.
The cathode and anodes of the diode sets are secured in position by three thin, flat copper stator terminal strips which apply little or no axial force to hold the diodes down during the soldering process and during alternator operation in the field. As the solder begins to liquify, the molten liquid flows under the diodes to cause the diodes to float up and even tilt. The diodes thus lose approximately 50% of the thermal conductivity to the heat sinks.
Placing the diodes deep within the wells restricts the cooling air from flowing around the diodes. The use of the same silicone insulating gasket between the heat sinks creates the same inherent failure mode as in U.S. Pat. No. 4,606,000.
Other patents, such as U.S. Pat. No. 3,959,676, describe the use of button diodes which require an insulated circuit board and a complex "U"-shaped stator terminal. Still other patents, such as U.S. Pat. No. 4,065,686, describe systems requiring precision holes and computerized presses to gently press-fit diodes into cavities because of the extreme sensitivity of the diodes to shock, stress and compression forces.
The new and novel invention described herein eliminates all of the above problems and reduces manufacturing costs and failures by using compression springs to apply a predetermined force to the diode. Three extra stator heat sinks are added with slotted air cooling fins and slots extending into the alternator's cooling air supply area. A copper positive heat sink with high conductivity and 50% more surface area rapidly transfers the heat generated out of the diodes, thus lowering the operating temperature by approximately 15%, as shown, for example, in FIG. 1.
The present invention also replaces the complex manufacturing process with a simple riveting and soldering operation to produce a rectifier having an extended life and which is simple to install in the alternator.